1. Field of the Invention
The present invention generally relates to a power supply system, and more particularly to a power supply system for optical network units used in Fiber In The Loop systems.
2. Description of the Prior Art
Several Fiber In The Loop (FITL) systems have already been suggested in the art. A typical FITL system has a construction, for example, comprising an exchange terminal, a remote terminal connected to the exchange terminal via an optical transmission path, and a plurality of optical network units connected to the remote terminal via an optical transmission path. A subscriber (customer) terminal is connected to the corresponding remote terminal via a subscriber line. In such a system, it is suggested to have a power supply system in which electric power for the optical network unit is supplied from the remote terminal. Development of an efficient power supply system is expected.
A description will now be given, with reference to FIG. 1, of a structure of an example of the FITL system. As shown in FIG. 1, the example of the FITL system comprises an exchange terminal 1, a remote terminal (RT) 2, a plurality of optical network units (ONUs) 3, a plurality of subscriber terminals 4, and a power supply unit 6. The exchange terminal 1 is connected with the remote terminal 2 via an optical transmission path. Each of the optical network units 3 is connected to the remote terminal 2 via an optical transmission path 7. Each of the subscriber terminals is connected to the corresponding optical network unit 3 via a subscriber line 5.
The exchange terminal 1 comprises a conversion unit 9 which performs an electrical to optical or optical to electrical conversion, and a path network (NW) 10 which performs an exchanging operation for a signal supplied to/from the conversion unit 9. Additionally, there is provided, between the conversion unit 9 and the path network 10, a multiplexer for multiplexing/demultiplexing the signal.
It is generally the case that the remote terminal 2 has a cross connecting function between the exchange terminal 1 and each of the optical network units 3. For example, in the remote terminal 3, a multiplexed optical signal is converted into an electric signal, and the electric signal is demultiplexed; desired lines are then connected by means of a time slot interchanging operation; the electric signal is multiplexed again and converted into an optical signal; the optical signal is then sent over an optical transmission path.
The optical network units 3 are placed relatively near the subscriber terminals 4. Conventionally, it is general to obtain electric power for the optical network units 3 from commercially available electric lines close to the optical network units 3. Such a system is described in Japanese Laid-Open Patent Applications No.62-59494 and No.62-128261. However, there is a problem in that if there are no electric lines available in the vicinity of an optical network unit, the optical network unit cannot be installed.
In order to eliminate the above-mentioned problem, there is suggested a system in which the remote terminal 2 is provided with a power supply unit 6 so that electric power is supplied from the remote terminal 2 to the optical network units 3 via power supply lines 7. For safety purpose, the power supply unit 6 is provided with a current cutoff unit 8 so that the electric current supplied to the optical network units 3 via the supply lines 7 is cut off when an excessive current is to be supplied to the optical network units 3 so as to protect the power supply unit 6 from overload.
FIG. 2 is a block diagram of the remote terminal 2, the optical network unit 3 and the power supply unit 6 shown in FIG. 1. The remote terminal 2 comprises conversion units 11 and 15, multiplexing/demultiplexing units (MUXs) 12 and 14 and a time slot interchanging unit (TSI) 13. The optical network unit 3 comprises a conversion unit 24, a multiplexing/demultiplexing unit 25, a plurality of subscriber terminal interfaces 26 and a power distribution unit 27. The power supply unit 6 comprises a current detecting unit 18, a power source 19 and a switch 20. The power supply line 7 connects the switch 20 of the power supply unit 6 to the power distribution unit 27 of the optical network unit 3. The conversion unit 15 of the remote terminal is connected to the conversion unit 24 of the corresponding optical network unit 3.
An optical signal input from the exchange terminal 1 (shown in FIG. 1) to the remote terminal 2 is converted into an electrical signal, and supplied to the multiplexing/demultiplexing unit 12. The electric signal is demultiplexed by the multiplexing/demultiplexing unit 12, and supplied to the time slot interchanging unit 13. The time slot interchanging unit 13 can substantially between any two desired lines of the multiplexing/demultiplexing units 12 and 14 by means of a time slot interchanging process. The multiplexing/demultiplexing units 14 and the conversion units 15 are provided to correspond to the number of optical network units 3. The signals multiplexed by each of the multiplexing/demultiplexing units 14 are then converted into optical signals by the conversion unit 15, and then sent to the respective optical transmission paths 22.
In each of the optical network units 3, the optical signal supplied via the optical path 22 is converted into an electrical signal by the conversion unit 24. The electrical signal is then demultiplexed by multiplexing/demultiplexing unit 25 so as to correspond to the subscriber terminal interfaces 26. The electrical signal is converted, by the subscriber terminal interface 26, to analog form in the case that the corresponding subscriber terminal is an analog device.
A signal input from the subscriber terminal to the optical network unit 3 is conversely processed so as to be sent to the optical path 22.
In the optical network unit 3, supply of a loop current for detecting a call signal from a subscriber terminal and sending of a call signal to a corresponding subscriber terminal are performed. Those operations require electric power. Additionally, electric power is also needed to operate the conversion unit 24 and multiplexing/demultiplexing unit 25. The electric power is supplied from the power source 19 of the power supply unit 6 via the current detecting unit 18 and the switch 20 by the power supply line 7. The electric power supplied via the power supply line 7 is distributed to each part of the optical network unit 3 by the distribution unit 27.
In the above-mentioned structure, when it is detected by the current detecting unit 18 that the electric current supplied to the optical network unit 3 exceeds a predetermined level, the switch is turned off so that the current supplied to the optical network unit 3 is cut off so as to protect the power supply unit 6 from overload. In order to achieve the above-mentioned overload protection system, the current detecting unit 18 and the switch 20 may be provided for each optical network unit 3.
In the above-mentioned conventional FITL system, when some subscriber terminals 3 connected to one optical network unit are in a communicating state and when a plurality of call-in signals and call-out signals are generated for the particular optical network unit 3 at the same time, there may occur a case where the current supplied to the optical network unit 3 from the power supply unit 6 is increased and finally exceeds the predetermined level. In such a case, the switch 20 is turned off in accordance with a detection signal supplied by the current detecting unit 20. Thereby, the power supply unit 6 can be protected from overload.
However, there is a problem in that the optical network unit 3 drops out of service when the electric current from the power supply unit 6 is cutoff, resulting in all communication channels corresponding to the optical network unit 3 being broken. In a case where a single common current detecting unit 18 and switch 20 combination is provided for all optical network units 3, a great number of currently established communication channels through a plurality of optical network units 3 are suddenly broken without warning.